Dispensers of hearing aids often fail to complete the sale of a hearing aid to a patient or client who could otherwise benefit from using one. One possible reason for such failure is that a patient or client may be unwilling from the start to purchase a hearing aid because he has no realistic way of knowing in advance how a hearing aid can help alleviate his hearing difficulties. Another possible reason is that a patient or client who actually does purchase a hearing aid may return it when his unrealistically high expectations were not met by the hearing aid in the outside world. Thus, a method and apparatus for demonstrating to patients and clients the potential benefits of hearing aids under acoustic real-world conditions is desirable.
In the last decade, microphones and signal processing used in hearing aids have become more and more sophisticated. Today, many models of hearing aids provide much more for the listener than simply the amplification of sound. In particular, directional microphones in hearing aids provide listeners with demonstrable improvements in speech intelligibility in noise. However, the United States Food and Drug Administration (FDA) has recently issued guidelines to the hearing aid industry, calling for dual-site, refereed clinical studies to substantiate claims made in hearing aid advertising beyond, simply, that a hearing aid amplifies sound. Such studies are to include evaluations of real-world subjective performance, such as by self-reporting by hearing aid users. But, at best, self-reporting procedures are time consuming and of limited reliability. A fast, reliable, objective apparatus and method of evaluating subjective, real-world performance of hearing aids is also therefore desirable.
Attempts to provide such a desirable apparatus and method, however, have failed to adequately address the above problems. For example, standardized bench tests of hearing aid performance have been developed providing industrial quality-control procedures, as mandated by the FDA (ANSI S3.22). However, the American National Standards Institute (ANSI) working group responsible for such tests (S3/WG48) has recognized that the current standards do not address the need to give clinicians ways to predict the efficacy of hearing aids in real use, even on a broad, general basis (i.e., for the average user). The ANSI working group is therefore developing standardized bench tests toward helping to fill that need. But, in any form, bench tests of a hearing aid can at best describe its average performance, that is, for the average hearing-aid user. Clinicians also seek, however, a fast, objective, and reliable method of showing that hearing aid products are effective for their clients on an individual basis. Thus, an objective method and apparatus for testing the real-world performance of hearing aids for individual users in a clinical setting is desirable.
In addition, an important aspect of hearing loss is one's ability to understand speech in the presence of masking noises and reverberation. Currently available clinical methods of assessing speech intelligibility in noise and reverberation generally rely on signal delivery systems that use either one or two earphones, or else one or two sound-field loudspeakers. Such systems may not present listening conditions to the ear which adequately exercise the auditory system in ways indicative of real-world function. Specifically, earphones, even if two are used, do not permit the listener to use hearing aids during testing. Nor do earphone tests account for the auditory effects of listening in a sound field. And one- or two-loudspeaker sound-field systems cannot surround the listener with background noise, as is the case in the real world, while presenting speech intelligibility tests.
Moreover, a principal goal of recent developments in hearing aid design has been to improve a hearing-impaired listener's ability to understand speech in noisy environments. Through the early 1990s, common experimental designs testing speech intelligibility in noise used either one loudspeaker for both noise and target signals, or else used one loudspeaker for the noise and another for the target signal. With such simple experimental setups, the background noise presented through the one loudspeaker may have consisted of sounds representative of real-life environments. However, the auditory experience of listening to such a one-loudspeaker presentation of background noise is far from that of actually being in a real-life noisy environment. Sound coming from one loudspeaker simply does not surround the listener as does sound in real-life noisy situations. Additionally, with such a presentation of background noise from only one loudspeaker, the binaural auditory system of a listener may not experience the same level or type of difficulty as it would in real-world noisy environments. In some cases, a subject's understanding of target speech tests could be accomplished more easily in the experimental environment than in real-life environments. In other cases, because the spatial and directional information of real-life acoustic environments is not present in the experimental environment (background noise does not surround the listener), a subject cannot fully use his natural signal-extraction mechanisms that rely on spatial and directional cues, thus making speech understanding more difficult than in real life. In short, single-or dual-loudspeaker signal-delivery systems used in research on speech intelligibility in noise do not provide an adequate representation of real-world adverse listening conditions, in part because the background noise does not surround the listener.
Within the last few years, an important development in hearing-aid design has been the addition of directional microphones. A directional microphone helps the listener understand speech in noise, because unwanted sounds coming from directions surrounding the listener are attenuated as compared to sound coming from directly in front of the listener. Therefore, the listener, when wearing a hearing aid equipped with a directional microphone, needs only to look at a talker to improve the signal-to-noise ratio (SNR) of the targeted speech signal produced by the hearing aid. An improved SNR translates into improved speech intelligibility for the signals received by a listener, as documented by many published scientific studies.
Attempts have been made to quantify improvements in speech intelligibility in noise for listeners wearing directional hearing aids. Sound systems used in such attempts have consisted of many problematic designs. One such system uses a single loudspeaker placed behind the listener to present background noise, while the targeted speech is presented in front of the listener. Such a system does not adequately document real-life performance, because, with directional microphones, the improvement in SNR observed when noise comes from a single, rear loudspeaker is not the same as when noise comes from directions surrounding the listener.
Multiple loudspeaker systems have thus been used to present background noise from directions surrounding the listener for the purpose of testing the performance of directional hearing aids. Such systems, however, have presented signals that are, for many reasons, not life-like.
Specifically, one such system presents uncorrelated Gaussian noise (spectrally shaped white noise) from several loudspeakers placed around the perimeter of the listening room. This system is successful at creating a purely diffuse sound field. But Gaussian noise (the same kind of noise as found between stations on an FM radio) is not indicative of real-world environments, because most real-life sound fields combine diffuse signals with direct signals.
Another such multiple loudspeaker system presents pre-recorded voices from multiple loudspeakers surrounding the listener. In this system, the presented, pre-recorded voices are recorded using microphones placed close to the talkers' mouths, in such a way as to remove from the recordings any acoustical conditions present in the recording environment. In other words, the recordings contained only the voices, and not the acoustical qualities of the recording environment. There are several problems with this system.
First, because the voices are recorded of talkers talking individually, usually while reading from a text, the presented speech sounds are not real conversations, and thus do not have many of the acoustical qualities that are present in real-life conversational speech.
Second because the loudspeakers are placed at a considerable distance from the listener (perhaps even beyond the “critical distance” of the listening room), the acoustical conditions in the listening room exert a substantial influence on the sounds received by the listener. Thus, even if representations of the acoustical conditions of the recording environment are present in the recordings of the individual voices presented over multiple loudspeakers, the acoustical conditions of the room used for playback override the acoustical representations of the recording environment, because the loudspeakers are too far away from the listener for the direct sound from the recordings to predominate.
And third, the signal from each loudspeaker is completely uncorrelated with the signals from each other loudspeaker, and therefore the total signal is not life-like. In real-life listening environments, which contain the acoustical qualities of the listening environment, the signals received by the listener from one direction are partially correlated with signals received from each other direction.
In any event, clinicians involved in the diagnosis and treatment of central auditory processing disorders have reported that these standard methods for testing the hearing abilities of patients who complain of having difficulty hearing in certain acoustic environments, do not always provide adequate information about the problems underlying the complaints. The reason for this may be that a patient's problems exist only under certain acoustic conditions encountered outside those afforded by currently available clinical tests. Thus, a method and apparatus that effectively places the patient under the same adverse listening conditions which are known to occur in the real world, and can therefore excite the reported problem, is therefore desirable.
It may be suggested that a system similar to entertainment “surround sound” systems may be used to address many of the above-mentioned problems. However, such entertainment systems are not suited for use in hearing and hearing aid assessment for many reasons. For example, in entertainment audio systems, the loudspeakers are located substantially distant from the listener, at or near the perimeter of a listening area that is accessible to multiple listeners. As with previous multiple-loudspeaker systems used in hearing and hearing-aid assessment, signals received by listeners from such entertainment audio systems contain a substantial contribution of the acoustical qualities of the listening environment. In any system that delivers signals containing the acoustical qualities of the listening environment as such, a given recording sounds somewhat different in different listening environments and has different acoustical qualities in each listening environment. Such systems, therefore, do not enable the desired standardization for hearing and hearing aid assessment.
In addition, entertainment audio systems are designed so that background noises presented to the listener enhance or support the reception of an entertainment event, such as a primary audio signal or a visual picture. In the real world, however, background noises presented to the listener do not enhance or support the reception of a primary audio signal or a visual picture. Instead, background noises disrupt or compete with the reception of such primary stimuli, resulting in conditions under which the reception of such primary stimuli breaks down. It is these real-world conditions that are desirable for hearing and hearing aid assessment.
It is therefore an object of the current invention to provide a sound reproduction method and apparatus which simulates or reproduces life-like acoustic environments for the purposes of testing or demonstrating, in a laboratory, a clinic, a dispensary or the like, the performance of hearing and/or hearing aids under conditions of real function.